A combination of a chain and sprocket as a power transmission system is widely employed in a motorcycle, bicycle or other various industrial machines. The shape and size of the chain and sprocket are defined in JISB1801 and JISB1802 of the Japan Industrial Standards.
To reduce the amount of material or the production cost, or to reduce the weight of a motorcycle and improve response during acceleration, a predetermined pattern of weight reduction holes are formed in the sprocket. A sprocket which is mounted on a rear wheel of a motorcycle is, in many cases, exposed to ambient atmosphere. Such holes have an effect on the aesthetics of the motorcycle. To this end, there have been proposed holes of various shapes and sizes.
As mentioned above, such holes are effective for reducing the weight of the sprocket to some extent. The holes can not, however, be formed at the outer periphery of the sprocket since it is necessary to maintain the strength and rigidity of the teeth. Thus, a further reduction in the weight of the sprocket is not possible. It should be noted that to improve response during acceleration, it is preferable to reduce the weight of the outer periphery of the sprocket. However, the weight of the outer periphery of the sprocket can not be reduced by the holes as mentioned above.
A chain is typically composed of two pin link plates and roller link plates extending parallel to each other and arranged in an alternate fashion, and mechanically connected by pins. This arrangement inevitably places limits on tensile strength. As such, the use of a narrower chain creates a problem as to strength. In contrast, the sprocket has no mechanical connection. Various materials have recently been proposed to increase the strength and rigidity of a sprocket. A sprocket made of such materials enjoys the same strength and rigidity if the thickness is reduced by one-half that of a conventional sprocket.
There may be employed a combination of a thin and lightweight sprocket and a conventional chain with a sufficient degree of strength. Such a combination may, however, create rattling in the thickness direction of the sprocket. Such vibration or rattling, it occurs, results in improper engagement of the sprocket with the chain, produces noise, and causes irregular wear of the sprocket and the chain and twisting of the sprocket and chain due to application of uneven loads.
It is, therefore, an object of the present invention to provide a sprocket which is thin, but enables smooth engagement of the sprocket with an associated chain, which provides a sufficient degree of torsional strength, and which has an aesthetically pleasing appearance.